1. Field of the Invention
The present invention relates to a golf ball having at least a pair of adjacent concentric solid layers, and particularly to a golf ball capable of providing a player with different feels on impact depending on the direction of an external force applied thereto when the golf ball is hit.
2. Description of the Related Art
Some golf balls have at least a pair of adjacent concentric solid layers. Examples of such golf balls include a three-piece golf ball having a two-layered solid core composed of a low-hardness inner core and a high-hardness outer core bonded onto the inner core, and a two-piece golf ball composed of a high-hardness solid core enclosed by a low-hardness cover. In such golf balls, a combination of the low-hardness inner core and the high-hardness outer core or a combination of the high-hardness solid core and the low-hardness cover refers to a pair of adjacent concentric solid layers.
In the conventional golf balls described above, the interface between adjacent concentric solid layers has a smooth spherical shape. Accordingly, an external force applied to a golf ball upon being hit is transmitted from an outer concentric solid layer to an inner concentric solid layer at a substantially constant rate irrespective of the direction of the applied force. That is, the ratio between an external force transmitted from the outer concentric solid layer to the inner concentric solid layer and an external force applied to the outer concentric solid layer is substantially constant. This is because a distance along which an external force is transmitted within the outer concentric solid layer varies little with the direction of an external force applied to the golf ball. Thus, in conventional golf balls, which have a spherical interface between concentric solid layers, provide a player with substantially the same feel on impact irrespective of the direction of an external force applied thereto.
For golf shots intended to impart a large backspin to a golf ball (a chip shot, for example), such as for an approach shot, a player performs a down-blow swing using a golf club having a large loft angle. Consequently, the club face of the golf club hits and rubs the surface of the golf ball downward on impact, resulting in the application of an external force to the golf ball substantially along a tangential direction (a tangent on the ball surface at a position hit by the club face). By contrast, for shots intended to give a golf ball a long travel distance, such as for a driver shot, a player performs an upper-blow swing or level swing using a golf club having a small loft angle. Consequently, the club face of the golf club hits the golf ball on impact such that it substantially faces the center of the golf ball, resulting in the application of an external force to the golf ball substantially along a normal direction (normal to the ball surface at a position hit by the club face).
Some golf balls having at least a pair of adjacent concentric solid layers as described above are designed to provide a soft feel on impact when a player performs a shot, such as an approach shot, intended to impart a large backspin to that golf ball. Examples of such golf balls include the aforementioned three-piece golf balls and two-piece golf balls.
However, when the player attempts a shot intended for a long travel distance, such as a driver shot, the aforementioned golf balls designed to provide a soft feel on impact for an approach shot or the like give a player dissatisfaction as described below. Since this type of conventional golf ball has a spherical interface between adjacent concentric solid layers, as already mentioned, an external force applied to an outer concentric solid layer is transmitted to an inner concentric solid layer at substantially the same ratio when the player makes a driver shot or the like, at which an external force is applied to the golf ball substantially along a normal direction, and an approach shot or the like, at which an external force is applied to the golf ball substantially along a tangential direction. As a result, during a driver shot, a deformation of the inner concentric solid layer, thus a deformation of the entire golf ball, becomes too large, resulting in too soft a feel on impact and thus giving a player an impression as if the golf ball had no core.